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From: lachman on 23 Apr 2010 00:57
jgharston writes:
>lach wrote:
>> ...and won't compile into an assembly subroutine of less than two
>> dozen instructions on a QL for both Julian & Gregorian dates,
>
>Dunno about 68000 code, but 6502 code here:
> http://mdfs.net/Info/Comp/6502/ProgTips/DayOfWeek

That code is slow & dirty. On the one hand it assumes: that
all dates are stored in non-standard fashion in 3 bytes, rather
than 4 as CYMD (but if there's an over-riding constraint to store
dates in 3 bytes, one may store the 1st digit of the century in
the upper 3 bits of D, and the 2nd in the upper 4 bits of M); and in
the alternative, that CY is to be pre-processed by subtracting 1900
before passing the result to register Y. The latter is cheating--
if that's acceptable, then one ought to cheat properly by looking up
the Century offset in the offset-table, summing it with D, and then
passing the sum to the subroutine when called. As 'month'<13, and
'century'>14 for Gregorian dates, both tables may be combined into
one. Century offsets may be found at:

http://de.wikipedia.org/wiki/Wochentagsberechnung#Jahrhundertziffer

This has the added benefit of the year register always being less
than 100, so that there's no need to test for Century 21. Further
improvements could be realised by rewriting the code to eliminate
the need to invert register A, due to A's total sum always being
less than 100. This will yield another benefit: to have the code
also work for dates in BCD. The code can thus be streamlined to
have a wider range of applicability, yet use fewer clock cycles.

>> additions it could return the julian day number instead; Then MOD7+1
>> to find the weekday number.
>
>You didn't ask for Julian number, you asked for day of week.

I didn't ask for the day of week either. I'm telling how to find
the weekday number in a different, if not better, way.
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